A Wide Area Network (WAN) which employs an Internet Protocol-Virtual Private Network (IP-VPN) technique or the like is generally used as a communication network between global bases.
In a case where a terminal present in a certain base communicates with a terminal present in another foreign base, the communication is performed via a line which connects a self base Local Area Network (LAN) to a domestic WAN, a line which connects the domestic WAN to a foreign WAN, and a line which connects the foreign WAN to another base LAN. In these lines, an available bandwidth is limited depending on contract bandwidths.
A Transport Communication Protocol (TCP) is generally used in communication between terminals. In the TCP communication, with respect to data which is transmitted by a transmission terminal, a reception terminal notifies the transmission terminal of an amount of the received data in a feedback manner. When the amount of the received data the notification of which is sent in a feedback manner does not increase, the transmission terminal determines that discarding is detected.
In addition, the transmission terminal manages a parameter called a window size (a size of data which can be transmitted even if a notification of reception thereof is not sent from the reception terminal) and varies the window size depending on a Round Trip Time (RTT) or whether or not discarding is detected.
The transmission terminal determines that a network is congested when the RTT increases or discarding is detected, and reduces a window size so as to indirectly reduce a transmission bandwidth, thereby preventing the network from being congested. In addition, it is determined that the network is vacant when the RTT decreases or there is no discarding, and the window size is increased so as to indirectly increase the transmission bandwidth, thereby effectively using a line bandwidth of the network. As above, the transmission bandwidth is greatly dependent on the RTT and a discarding rate in the communication using the TCP.
As a transmission bandwidth control technique such as the TCP communication, there is Available Bit Rate (ABR) of Asynchronous Transfer Mode (ATM) disclosed in NPL 1 or communication using a technique disclosed in PTL 1. In this communication, a reception terminal notifies a transmission terminal of information of a reception bandwidth or whether or not there is congestion in a feedback manner, and the transmission terminal controls a transmission bandwidth on the basis of the information the notification of which is sent in a feedback manner.
In addition, there is a technique disclosed in PTL 2 as the transmission bandwidth control technique such as the TCP communication. In communication using this technique, control is performed such that a sum total of a transmission bandwidth of data body, a transmission bandwidth of error encoding data, and a retransmission bandwidth of discarded data becomes a constant value. When the retransmission bandwidth increases, the transmission bandwidth of the error encoding data decreases.
Further, there is a technique disclosed in PTL 3 as the transmission bandwidth control technique such as the TCP communication. In this technique, a bandwidth before being input to and a bandwidth after being output from a communication device which performs bandwidth limitation are compared, and a bandwidth limitation value is varied when a packet discarding bandwidth related to packet discarding due to a bandwidth limitation exceeds a specific value.